The present invention relates to a gas turbine combustor. More specifically, the present invention relates to a low NOx combustor having the capability of burning lean mixtures of gaseous fuel.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Fuel was introduced into the primary combustion zone by means of a centrally disposed fuel nozzle. Additional air was introduced into the combustor downstream of the primary combustion zone so that the overall fuel/air ratio was considerably less than stoichiometric--i.e., lean. Nevertheless, despite the use of lean fuel/air ratios, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range of firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of rich fuel/air mixtures in the primary combustion zone resulted in very high temperatures. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios reduces NOx formation. However, achieving such lean mixtures requires that the fuel be widely distributed and very well mixed into the combustion air. This can be accomplished by pre-mixing the fuel into the combustion air prior to its introduction into the combustion zone.
In the case of gaseous fuel, this pre-mixing can be accomplished by introducing the fuel into primary and secondary annular passages that pre-mix the fuel and air and then direct the pre-mixed fuel into primary and secondary combustion zones, respectively. The gaseous fuel is introduced into these primary and secondary pre-mixing passages using cylindrical fuel spray tubes distributed around the circumference of each passage. A combustor of this type is disclosed in U.S. Pat. No. 5,394,688 (Amos), hereby incorporated by reference in its entirety.
The presence of the cylindrical fuel spray tubes in the pre-mixing passages creates turbulence in the air flow immediately downstream of the tubes. Such turbulence is not undesirable since it aids in mixing the fuel and air. However, the recirculation associated with such turbulent zones can cause the fuel spray tube to act as a flame holder, so that combustion occurs prematurely in the pre-mixing passage, rather than in the combustion zone as intended. This situation can cause damage to the fuel tubes and the liners forming the pre-mixing passage.
It is therefore desirable to provide a lean burning gas turbine combustor capable of introducing fuel into a pre-mixing passage with sufficient turbulence to provide mixing but without creating re-circulation zones that could act as flame holders.